Changes in the content of γ-linolenic C18:3 (n-6) and stearidonic C18:4 (n-3) acids in developing seeds of viper's bugloss Echium vulgare L.
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Changes in the composition of fatty acids (FA) were determined in lipid extracts isolated from developing ovaries containing ovules and developing seeds of Echium vulgare L. The samples were collected successively over 20 days beginning with the first day after flowering. The contents of the n-6 FA family members, i.e., γ-linolenic (GLA) (C18:3) and linoleic (LA) (C18:2) acids changed in a parallel manner and reached the maximum of 13.9% and 24%, respectively, on the 12th day, after which they fell systematically down to 8.6% and 18.2%, respectively, on the 20th day after flowering. Starting with day 13, the content of α-linolenic acid (ALA) (C18:3 n-3) begins to grow intensively, from 24.2% to 39.3% on the 20th day after flowering. The increase in the content of stearidonic acid (SDA) (C18:4 n-3), up to 10.5% on the 20th day after flowering, occurred steadily as the seeds developed, and was independent of the changes in the content of GLA and LA. The pattern of changes in the content of SDA, GLA, LA and ALA during the development of seeds, and the occurrence of SDA in the seed oil of other plants, demonstrate that the biosynthesis of SDA in the seeds is critically dependent on the presence of ALA. The above condition indicates that SDA biosynthesis in the seeds of Echium vulgare follows the scheme LA → simultaneous, competitive, action of Δ6 and Δ15 desaturases, leading to the formation of GLA and ALA, respectively, and then ALA (Δ6 des) → SDA. The biosynthesis according to the scheme: GLA (Δ15 des) → SDA is highly unlikely.
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